Such contactlessly working measuring devices are applied in a large number of branches of industry, e.g. in the processing industry, in the chemistry industry and in the foods industry.
Typically, the fill level measuring device is mounted above the fill substance, and its antenna is pointed toward the fill substance.
For determining travel times, all known methods can be applied which enable relatively short distances to be measured by means of reflected microwaves. The best-known examples are pulse radar and frequency-modulated continuous wave radar (FMCW radar).
In the case of pulse radar, short microwave pulses are periodically transmitted, which are reflected by the surface of the fill substance and received back after a distance-dependent travel time. Based on the received signal, an echo function is derived, which shows received signal amplitude as a function of time. Each value of this echo function corresponds to the amplitude of an echo reflected at a particular distance from the antenna.
In the case of the FMCW method, a microwave signal is continuously transmitted, which is periodically linearly frequency modulated, for example, according to a sawtooth function. The frequency of the received echo signal has, consequently, in comparison to the instantaneous frequency which the transmission signal has for the point in time of receipt, a frequency difference, which depends on the travel time of the microwave signal and its echo signal. The frequency difference between the transmission signal and received signal, which can be gained by mixing of both signals and evaluation of the Fourier spectrum of the mixed signal, thus corresponds to the distance of the reflecting area from the antenna. Additionally, the amplitudes of the spectral lines of the frequency spectrum gained by the Fourier transformation correspond to the echo amplitudes. This Fourier spectrum, consequently, in this case represents the echo function.
From the echo function, at least one wanted echo is determined, which corresponds to the reflection of the transmission signal off the surface of the fill substance. In the case of known propagation velocity for the microwaves, from the travel time of the wanted echo, there is directly yielded the distance which the microwaves travel on their way from the measuring device to the surface of the fill substance and back. On the basis of the installed height of the fill level measuring device over the container, the sought fill level can be directly calculated.
There are, however, a large number of applications, in the case of which this form of fill level measurement is insufficient.
An example of this is presented by fill level measurements in storage containers for bulk goods. Bulk goods form, as a rule, a bulk goods cone. The above-named classical fill level measurement delivers here the fill level in a particular region of the bulk goods cone predetermined by the antenna position and its orientation; however, a more exact determining of the fill substance volume is not achievable thereby. In these cases, so-called multipoint measurements are regularly performed today. In such case, a number of fill level measuring devices are arranged next to one another over the fill substance, and the fill levels in the individual regions in the container registered by the respective measuring devices are determined. The use of a number of fill level measuring devices is, as a rule, quite expensive and complicated. Alternatively, for this purpose, a fill level measuring device can be equipped with a plurality of antennas arranged at different locations over the fill substance, and, for example, these antennas are switched in individually via electronic switches arranged in the field. The use of a plurality of antennas which can be switched in via electronic switches is, in contrast to the above, indeed cost effective; it has, however, the disadvantage, that these switches, arranged, as a rule, directly on the antenna in the field, must be supplied with energy. This is not only complicated, but also represents a safety risk, especially in applications in which, for reasons of explosion protection, special safety precautions must be followed.
Further examples are applications in the case of which there are present in the container disturbances, e.g. stirring mechanisms or other installed objects, off of which the transmitted microwave signals are likewise reflected. In this case, the echo signal recorded with the fill level measuring device contains both the wanted echo to be traced back to a reflection off the surface of the fill substance, as well as also disturbing echoes to be traced back to reflections off the disturbances. Accordingly, it is very difficult—or, under certain circumstances, even impossible—to ascertain based on the echo signal the wanted echo sought, and therewith the fill level to be measured. For overcoming this problem, so-called multilobe measurements are frequently performed. In such case, the microwave signals are transmitted into the container in a plurality of transmission lobes with different orientations. The transmission lobes are, in such case, oriented, for example, in such a manner, that each transmission lobe reaches the fill substance. The echo signals of the individual transmission lobes are recorded, and, based on the known orientation of the different transmission lobes, additional information is gained, on the basis of which the wanted echo contained in all echo signals can be determined much more exactly and reliably. Examples for this are described in EP 1 431 724 A1. Described, among other things, is how to ascertain the wanted echo based on the amplitudes of the individual echo signals. While the amplitude of the wanted echo is, angle corrected, the same in all echo signals, the disturbing echoes in the different echo signals have different amplitudes due to the different orientations of the associated transmission lobes.
Both in the case of multipoint measurement, as well as in the case of multilobe measurement, it is necessary to transmit microwave signals into the container on different signal paths, and to evaluate their echo signals separately from one another.
For this, a number of parallelly operated transmitting and/or receiver arrangements and/or electronic switches can be applied. In DE 10 2004 034 429 there is described an example for this, which is applied in the automobile industry as a distance sensor. There, a microwave generator is successively connected via a switch to different transmission antennas, and to each of the receiving antennas is connected a separate receiving branch, via which the echo signal taken up by the respective receiving antenna is taken up and fed to a signal processing system. The associating of the individual measuring signals to the individual signal paths occurs here via the respective switch positions and the separate receiving branches.